1. Field of the Invention
The present invention relates generally to pointing-devices used in conjunction with digital computer displays, and, more particularly, to small touch-pads which, in response to a finger's movement across a touch-pad's active area, cause motion of a cursor across a computer's display screen.
2. Description of the Prior Art
Pointing-devices for controlling a cursor on a digital computer's display screen are essential for using a computer that employs a graphic user interface ("GUI"). Various different types of pointing-devices are available such as mice, trackballs, joysticks, digitizer tablets and touch-pads. Each of these different devices exhibits certain limitations.
For example, operating a mouse requires an appreciable amount of free area on a relatively smooth work surface immediately adjacent to the computer. Sliding, i.e. translating, a mouse across such a work surface rolls a ball that is secured within the mouse, and that contacts the surface. Rolling of the ball within the mouse effects a corresponding movement of the cursor across the display screen. Moreover, a computer program that receives the mouse's output signal may filter the mouse's signal to provide special effects. For example, the same translation of a mouse may move the cursor a greater or lesser distance across the computer's screen depending upon the speed of the mouse's translation. However, even with such filtering and even with an appreciable amount of free work surface area, achieving a desired cursor movement frequently requires lifting the mouse and moving it through the air without touching the work surface.
A trackball essentially is a mouse turned upside down. Consequently, rather than rolling a ball by translating the trackball's base across a surface, the trackball's base remains fixed and one rolls the ball directly with a finger. Consequently, a trackball enjoys an advantage over a mouse in that it requires only a fixed amount of space on a desk, or in a laptop or notebook personal computer. However, a trackball experiences problems with contamination because it must have an upward facing opening around the ball through which dust particles may enter its mechanism. Trackballs may also experience contamination problems if they are manipulated by a dirty finger.
A joystick is an elongated member that usually protrudes upward from a fixed base. A joystick converts a displacement of the elongated member from a pre-established neutral position into a continuous movement of the cursor displayed on a computer's screen. Consequently, a displacement of the joystick does not provide absolute control over the cursor's position as does the movement of a mouse or trackball. Rather, at best a joystick controls only the direction and speed of the cursor's movement. Therefore, several successive joystick displacements may be required to position a cursor at a desired location on a computer's screen.
As contrasted with a mouse, a trackball, or a joystick; a digitizer tablet permits immediately specifying, usually using a special stylus, an exact position at which a cursor is to be located on a computer's display screen. However, the one-to-one correlation between positions on a digitizer tablet's working surface and positions on the computer's display screen requires that an adequately high resolution digitizer be a physically large device. Consequently, generally a digitizer tablet is unsuitable for use with a laptop or notebook personal computer.
Particularly for laptop or notebook personal computers, touch-pads alleviate many of the problems experienced with other types of pointing-devices. Touch-pads are small digitizer tablets that, similar to a mouse or trackball, provide relative rather than absolute control over a cursor's position usually in response to a finger's movement across the touch-pad's active area. Similar to a trackball, touch-pads occupy only a small, fixed amount of work surface area. Moreover, a touch-pad may be sealed so it doesn't suffer from the contamination problems generally experienced by trackballs. However, because a touch-pad is physically small, effecting large cursor movements across a computer's display screen may require several successive finger strokes across the touch-pad's active area. To address this particular limitation of touch-pads, U.S. Pat. No. 5,327,161 ("the '161 patent"), which issued on an application filed by James D. Logan and Blair Evans, discloses a touch-pad which, similar to a joystick, causes a cursor on a computer's display screen to continue moving in a pre-established direction even though finger movement across the touch-pad's active area halts. This patent discloses that continued cursor motion occurs if a finger moving across the touch-pad's active area enters a pre-established border area at the perimeter of the active area. Alternatively, this patent discloses that continued cursor motion can occur upon activation of a mechanical "drag switch," disposed beneath the touch-pad, in combination a finger movement across the touch-pad's active area.
A limitation of the techniques for simulating a large touch-pad active area disclosed in the '161 patent are that inadvertently entering the border area, or inadvertently pressing too hard on the touch-pad, automatically triggers continued cursor motion. Consequently, at times the touch-pad disclosed in the '161 patent may exhibit difficulty in positioning a cursor analogous to the difficulty sometimes experienced with a joystick. Moreover, dedication of the touch-pad's border area for sensing only continued cursor motion reduces the amount of touch-pad active area that provides relative cursor positioning.